The present invention is generally directed to an apparatus for determining spectral absorption by a particular substance in a fluid. Specifically, the present invention is directed to a probe apparatus for sensing arterial pulse and blood oxygen saturation in a patient.
A system is needed for non-invasively monitoring pulse and arterial blood oxygen saturation in a patient. Along with blood pressure, pulse rate and blood oxygen saturation are routinely monitored in a patient undergoing surgery or receiving emergency medical treatment. Blood pressure and pulse rate provide essential information about the functioning of the patient's cardiovascular system. Blood oxygen saturation provides information about the patient's ventilation; thus, blood oxygen saturation is a key indicator of the patient's health, whether the patient is human or another animal.
The chemical composition of blood, including oxygen saturation, can be determined by drawing blood samples and employing well-known chemical analysis techniques. However, a non-invasive technique, in which no blood is drawn, is preferred, since drawing blood from a patient adds additional trauma to the patient. Also, analysis of the blood can be an expensive and time consuming procedure and may require elaborate chemical analysis equipment. Such time and equipment may not be readily available, particularly in the context of a medical emergency at a site remote from a hospital or other health care facility. A better technique uses a non-invasive probe which may be disposed after one or a few uses and is simple and inexpensive to manufacture and use. Using such a non-invasive probe yields routinely required information, such as pulse and blood oxygen saturation, by a simple, routine and portable measurement technique.
One method for non-invasively measuring the blood oxygen content of a patient involves monitoring the color of the blood, as disclosed in U.S. Pat. No. 2,414,747 to Kirschbaum for "Method and Apparatus For Controlling the Oxygen Content of the Blood of Living Animals." However, Kirschbaum's sensing apparatus is linked to an additional oxygen administering apparatus which limits its portability. Kirschbaum makes no provision for a disposable probe. Additionally, Kirschbaum's probe must be recalibrated for each use to account for differences in each patient and differences in the lamps used in the probe.
Use of red and infrared light sources in an oximeter is disclosed in U.S. Pat. No. 2,640,389 to Liston for "Oximeter." Liston's light sources are gas tubes containing neon and argon which emit light when excited electrically. Such tubes limit the portability and ruggedness of Liston's system. Also, Liston's probe is not practically disposable since the tubes are relatively expensive to produce.
A similar technique using red and infrared light in an oximeter is disclosed in U.S. Pat. No. 3,412,729 to Smith for "Method and Apparatus for Continuously Monitoring Blood Oxygenation, Blood Pressure, Pulse Rate and the Pressure Pulse Curve Utilizing an Ear Oximeter as Transducer." Smith also uses the data derived from the probe to provide blood pressure information. The oximeter of Smith uses a light bulb as a source of red and infrared energy. Use of an expensive, fragile element such as a light bulb limits the ruggedness and disposability of the Smith device and renders it unsuitable, for example, for emergency medical treatment at remote locations.
Infrared light has also been used to measure the concentration of blood analytes. As disclosed in U.S. Pat. No. 4,882,492 to Schlager for "Non-invasive Near-infrared Measurement or of Blood Analyte Concentrations," near-infrared light having a wavelength greater than 1800 nanometers (nm) is directed into a body part of a patient and sensed to determine the concentration of analytes such as glucose in the blood. As is well known in the art, however, determination of oxygen saturation is preferably achieved using both red and infrared light energy at wavelengths less than 1000 nm.
The probes of Kirschbaum, Liston, Smith and Schlager are preferably attached to the ear of a patient. U.S. Pat. No. 4,865,038 to Rich et al. for "Sensor Appliance for Non-invasive Monitoring" discloses a flexible probe which may be removably attached to a finger or other appendage. Such a probe has the advantages of providing the ability to conform to the appendage while maintaining secure attachment thereto. For medical procedures involving the head and neck, the ear probe could be inconvenient and create interference with access to the head and neck. The ability to attach the probe to a finger, toe or other appendage provides added flexibility and convenience, allowing the probe to be kept from interfering with other procedures. U.S. Pat. No. 4,825,872 to Tan et al. for "Finger Sensor for Pulse Oximetry System" discloses a similar probe which is retained on the finger by means of expansive side panels which expand and contract appropriately to engage the finger.
Rich et al. also disclose the use of solid state devices to generate the red and infrared light energy directed into the patient's blood, and solid state optical sensors to detect light not absorbed by the blood. Specifically, semiconductor light emitting diodes (LEDs) are used because they provide such advantages as small size, ruggedness, ready interface with other solid state circuitry used in the oximeter, long life, and an output having a wavelength that is stable over time.
As is well known in the art, the transmission of light having a wavelength of approximately 660 nm (i.e., red light) through blood is strongly affected by the amount of oxygenated hemoglobin present in the blood. As also known in the art, the transmission of light having a wavelength of approximately 940 nm (i.e., infrared light) is not substantially affected by the amount of oxygenated hemoglobin present. By using these phenomena and shining red light and infrared light through the flesh of a patient, the percent saturation of oxygenation in the patient's blood can be determined.
An oximeter according to the present invention determines blood oxygen saturation based on the intensity of light received by an optical sensor and the known wavelength of an LED. The wavelength of the absorbed light defines an extinction coefficient which the oximeter uses in calculating blood oxygen concentration in a manner known in the art. Consequently, the oximeter must be calibrated to the wavelength of the LEDs employed in the probe. However, different wavelengths of LED light require coefficients having different values be employed for determining blood oxygen concentration. That is, if a probe with a first LED having a first wavelength is replaced by a probe with a second LED having a different second wavelength, the oximeter must use a different coefficient in calculating blood oxygen saturation. An oximeter Using such a probe must be recalibrated, therefore, whenever a probe is changed, in order to maintain consistency of measurement and accuracy of results.
U.S. Pat. No. 4,700,708 to New for "Calibrated Optical Oximeter Probe" addressed the need to recalibrate an oximeter when disposable probes are used. New discloses the technique of including in the probe a resistor as an encoding means; the value of the resistor corresponds in a predetermined manner to the wavelengths of the red and infrared LEDs used in the probe. When the probe is manufactured, the LEDs are tested to determine their wavelengths and the appropriate encoding resistor is chosen according to a table. When the probe is connected to the oximeter, a constant current source contained in the oximeter passes current through the resistor thereby enabling the oximeter to read the resistor's value. The New oximeter then uses a corresponding look up table located in semiconductor memory in the oximeter to determine the appropriate wavelength of the LEDs in the probe and to select the requisite extinction coefficients. By using inexpensive components such as LEDs and a resistor, and by eliminating the need to recalibrate the oximeter when the probe is replaced, New provides a truly disposable probe.
The LEDs described by New are selected from batches having only generally known characteristics. Specifically, the wavelength tolerance of the LEDs is large relative to the range permitted by the oximeter. For example, the manufacturing tolerance of the LEDs might be .+-.20 nm. That is, a LED specified by the manufacturer as having a wavelength of 660 nm actually has a wavelength somewhere in the range from 640 nm to 680 nm. To correctly determine the appropriate extinction coefficients and thereby yield correct measurements, the oximeter must effect calculations based on the actual LED wavelengths within a tolerance of less than 5 nm. Consequently, the use of the resistor to encode the wavelength value is a necessary step in providing a disposable probe. Without the resistor present to encode the LED wavelengths, the New oximeter does not function. To minimize the manufacturing cost of the oximeter probe, New provided for the use of low cost LEDs with a large manufacturing tolerance and a resistor to encode the exact values of the LED wavelengths within their tolerance ranges.
Many present oximeter systems typically do not rely on encoding the LED wavelength values. Manufacturing techniques for LEDs have improved so that LEDs having a tight tolerance--as low as .+-.2 nm--are inexpensively available. Consequently, the encoding and look up table disclosed in New are not required in competing oximeter systems. Competing systems specify a center value for the LED wavelengths, as for example, 660 nm and 940 nm, and each conforming probe is then supplied with LEDs having those wavelengths.
In addition, the oximeter probe disclosed by New has several shortcomings. To keep manufacturing cost low, the encoding resistor has a large tolerance and a large temperature coefficient of resistance. At extremes of temperature, the encoding resistor will vary considerably from its nominal value, creating the possibility that incorrect extinction coefficients will be read from the look up table by the oximeter. Oximeter systems are used not only in the controlled environments of operating rooms but, also, in the emergency medical treatment context, in situations of environmental temperature extremes. Consequently, the operating temperature of such a system may range from well below zero degrees Fahrenheit to well over one hundred degrees Fahrenheit. Under such extremes, a more precise means of interfacing the probe to the oximeter is needed.
Also, the probe disclosed by New is subject to noise which may cause incorrect readings. Low frequency noise may be misinterpreted by the oximeter to be pulsatile information. Such noise may have its source in radio frequency radiation propagated through the environment, as from other nearby test equipment, or as a motion artifact, due to movement of the bodily appendage to which the probe is attached.
A probe which avoids the encoding technique disclosed by New also provides manufacturing economies. The use of tight-tolerance LEDs eliminates the need to maintain multiple stocks of different wavelength LEDs. A single bin of devices can be used, reducing storage costs. The necessary step of testing LED wavelengths is eliminated. In addition, the step of matching a resistor to the LED wavelength is eliminated. Using precision LEDs and eliminating the encoding resistor greatly reduces manufacturing cost and complexity.
Accordingly, there is a need for an oximeter probe which can accurately interface with multiple types of oximeter systems, including those which use an encoding resistor and those which specify a center wavelength value, as well as providing precise measurement characteristics in extreme as well as nominal environments, has reduced sensitivity to noise interference, and which is inexpensive enough to manufacture so as to be economically disposable after one or a few uses.